The present invention relates to an air filter used in a high efficiency air cleaning apparatus, such as a clean room, a clean bench, a storage means (stocker), etc., for removing gaseous inorganic and/or organic impurities contained in the atmosphere in the high efficiency air cleaning apparatus. The invention also relates to a method for manufacturing the air filter and further relates to the high efficiency air cleaning apparatus, the device or the like provided with the filter manufactured according to the method.
In these days, when semiconductors and LCD panels are manufactured, a high efficiency air cleaning apparatus is generally used. For instance, the production line for a semiconductor from a bare wafer to 1 megabyte DRAM chip, includes about 200 steps, and the production line to produce 9.4-inch TFT type LCD panel from solid glass includes about 80 steps. In these production lines, it is difficult to transfer wafers and glass substrates consecutively and continuously to next process. For instance, in the TFT-LCD production line, it is not rare to see half finished products, on which a predetermined circuit is already formed, forced to be kept inside of a carrier or a stocker, while being exposed to the inside atmosphere for several to several tens of hours until they are transferred to the next manufacturing step.
As stated above, when a semiconductor substrate or a LCD substrate is kept in an atmosphere of an ordinary clean room for a long time, gaseous impurities contained in the room would be deposited on the surface thereof. Recently, acid substances, basic substances, organic substances and various dopants are considered to exist in such clean room in a gaseous state and these substances give an ill influence to the performance of semiconductors or LCD panels when they are deposited on the silicon wafer surface used for semiconductor production, or on the glass substrate surface used for the LCD panel production.
According to an article titled xe2x80x9cForecast of Airborne Molecular Contamination Limits for the 0.25 Micron High Performance Logic Processxe2x80x9d of Technology Transfer #95052812A-TR publicized by SEMATECH on May 31, 1995, these acid substances, basic substances, organic substances and dopants are called chemical contaminants and respectively defined as follows.
xe2x80x9cacid substancexe2x80x9d: a corrosive substance to react chemically like an electron acceptor. (hydrofluoric acid HF, sulfur oxides SOx, nitrogen oxides NOx, etc.).
xe2x80x9cbasic substancexe2x80x9d: a corrosive substance to behave chemically like an electron donor. (ammonia NH3, amine, etc.).
xe2x80x9corganic substancexe2x80x9d: a substance having a boiling point higher than normal temperature under normal pressure, condensing on a cleaned surface. (siloxane, phthalate, HMDS, BHT, etc.).
xe2x80x9cdopantxe2x80x9d: chemical element to give ill influence to an electrical performance of the semiconductor device. (Boron B, phosphorus P).
Table 1 is a list of the allowable concentration (ppt) of the chemical contaminants required for 0.25 xcexcprocess (after ""98), which is disclosed in the article entitled xe2x80x9cForecast of Airborne Molecular Contamination Limits for the 0.25 Micron High Performance Logic Processxe2x80x9d of Technology Transfer #95052812A-TR published by SEMATECH (U. S. A) on May 31, 1995. The value in percentage shown in the bottom of the allowable concentration (ppt) shows the reliability of each allowable concentration value. The table shows the allowable concentration of the chemical contaminants in clean space of four semiconductor production processes having serious chemical contamination problems. According to one example of the actual measurement of various contaminants contained in the atmosphere of the ordinary clean room not provided with any chemical protective measure against gaseous contaminants, it is reported that acid substances of about 100 ppt-1,000 ppt, basic substances of about 1,000 ppt-10,000 ppt, organic substances of about 1,000 ppt-10,000 ppt, and dopants of about 10 ppt-100 ppt, respectively are contained in such atmosphere.
When concentration of the chemical contaminants in the atmosphere of the ordinary clean room not provided with any chemical protective measure against gaseous contaminants, and the allowable concentration of the chemical contaminants in Table 1 are compared, it is found that severe controls are required for the processes stressed by underlines under the numerals of the allowable concentrations (ppt). That is to say, with regard to acid substances, they should be controlled to less than 180 ppt in the salicidation process, and less than 5 ppt in the contact formation process. As for basic substances, they should be controlled to less than 1 ppb, in the photolithographic process. With respect to the dopant, it should be controlled to less than 0.1 ppt for the pre-gate oxidation process. As to organic substances, they should be controlled to less than 1 ppb in the pre-gate oxidation process, and less than 2 ppb in the contact formation process.
Such impurities as gaseous acid substance, basic substance, organic substance, and dopant shown in Table 1, cause problems if contained in various high efficiency air cleaning apparatus for production of semiconductor substrate and glass substrate, such as clean room, cleanbench and clean chamber, various scale of high efficiency air cleaning apparatus such as stocker for keeping clean products, and local high efficiency air cleaning apparatus called mini environment.
Among them, dopant shows a chemical behavior resembling acid substance as water-soluble borate compound or phosphorous compound, and a filter having a capability to adsorb and remove gaseous acid substance can adsorb and remove the dopant. As apparent from Table 1, simultaneous removal of organic substance and dopant in the pre-gate oxidation process, and simultaneous removal of acid substance and organic substance are required respectively.
Though not mentioned in Table 1, gaseous contaminants generated in the photolithography process are HMDS (hexamethyl disilazane) and its decomposition product besides ammonia. HMDS is a lipophilic substance to coat on a wafer to improve affinity of a litho-film to a silicon wafer and very easy to adhere to a surface (wafer, lens, glass etc.). HMDS is hydrolyzed in a few days to gasify into ammonia and trimethyl silanol. When trimethyl silanol adheres to a lens or mirror thereby making the surface thereof blur, it causes an exposure trouble during the exposure process. KrF laser exposure (248 nm) is used in a device having 0.25 xcexcm in line width, and the KrF laser exposure is considered to be still used in a gigabyte-capable device having 0.18 xcexcm in line width which is expected to start mass production from the year of around 2000.
The clouding of lens by HMDS or trimethyl silanol gives a fatal effect. Both HMDS and trimethyl silanol are organic substances with no ionization and did not draw much attention at the time of 1997. In a photolithography process for manufacturing the gigabyte-capable device having 0.18 xcexcm in line width, however, simultaneous removal of base and organic substance is necessary.
Of the four chemical contaminants, three kinds of contaminants, an acid substance, a basic substance, and dopant, are soluble and they are apt to give rise to ion-exchange reaction and neutralization reaction. As a means for removing these three kinds of chemical contaminants from the air in a clean space, there has been a method for removing them by dissolving into aqueous solution using a wet rinsing (scrubber rinsing) and a chemical adsorption method using so-called chemical filter such as ion-exchange fiber and a chemical impregnated activated charcoal. On the other hand, among four kinds of chemical contaminants, majority of organic substances, do not dissolve in water and a physical adsorption method using activated charcoal has been used to remove them from the air in a clean space.
Gaseous inorganic impurities of acid substance, basic substance and dopant have been removed hitherto by three methods, wet type rinsing, ion-exchanging, and chemical impregnated activated charcoal, as described above.
The wet type rinsing is a method to dissolve and remove the acid substances, basic substances, and dopants by spraying droplets.
A cylindrical case in which particulate activated charcoal impregnated with chemicals is filled up in a designated case is known as the most simple configuration of a chemical filter using activated charcoal impregnated with chemical. Chemical filters of other types are also known. For instance, one type of chemical filters are made in the form of a felt by interweaving chemical-impregnated fibrous activated charcoal with an organic binder of low melting point polyester or polyester non-woven fabric, and the other type of chemical filters are formed in the shape of a block or a sheet by firmly binding particulate activated charcoal impregnated with chemicals to an air permeable urethane foam or a non-woven fabric with a proper adhesive. These chemical filters adsorb and remove gaseous acid substances and gaseous basic substances by neutralization reaction with the impregnating chemicals.
A chemical filter using an ion exchange fiber performs ion-exchange and removal of various ions, that is, acid and/or basic impurities contained in the air with a filter made of an acid cation exchange fiber and a basic anion exchange fiber in the form of a non-woven fabic, a sheet, or a felt as its basic filter media.
However, in the case of the wet type air rinsing system, an atomizer requires a rather large amount of initial cost for installation thereof and it also requires a considerable amount of running cost for the high pressure loss of spraying, which can not be overlooked. Furthermore, a clean room used for the production of a semiconductor element (LSI) or LCD, is kept at 23xc2x0 C. to 25xc2x0 C. with relative humidity of 40% to 55%. Accordingly, when the air in the clean room is circulated during wet rinsing, it becomes necessary to adjust again the temperature and humidity in the air after spraying to manage the temperature drop and the humidity increase after spraying. In addition, a means to remove droplets remained in the air after spraying is required in the downstream side of the atomizer. (the so-called carry-over issue). Furthermore, there arises problems peculiar to water treatment, such as treatment of washing water used circulatedly in the atomizer, preventing bacteria from being generated and/or condensing and separating dissolved contaminants.
The so-called chemical filter, utilizing chemical-impregnated activated charcoal and ion-exchange fiber, has the following disadvantages. First, the disadvantage, common to both types of chemical filters, can be cited. For instance, as for a clean room in which the ceiling is served as an air outlet of the clean air, it is extremely effective for removal of gaseous impurities in the air atmosphere in the clean room to install the chemical filter in the upstream side of the particulate filter installed on the ceiling. However, activated charcoal is a flammable substance specified by the Fire Service Act and the ion-exchange fiber is made of very ignitable material, which requires intensive caution for fire. Accordingly, it is difficult, from a viewpoint of disaster prevention, to install a chemical filter using the activated charcoal or the ion-exchange fiber.
Furthermore, a chemical filter using the chemical-impregnated activated charcoal has the following disadvantage. A conventional cylinder case type chemical filter has an advantage of high adsorption efficiency of impurities, but has a disadvantage of a high pressure loss (ventilation resistance). The conventional chemical filter in the form of a felt or a sheet has an excellent air permeability, and its adsorption efficiency is not so inferior compared to the cylinder case type. However, gaseous impurities desorbed from the constituents (for instance, non-woven fabric, organic binder, etc.), adhesives which fixes charcoal to the sheet (for instance, neoprene series resin, urethane series resin, epoxy series resin, silicon series resin, etc.), sealant to use for fixing filter materials to the surrounding frame (for instance, neoprene rubber, silicon rubber, etc.) are mixed with the air once cleaned by passing through the chemical filter and would give a negative influence to the manufacture of the semiconductor device.
It should be noted that, the chemical filter using chemical-impregnated activated charcoal in the form of a felt or a sheet removes very small quantity of acid or basic impurities on the order of ppb contained in the clean room, and dopant of ppt order, but it mixes gaseous organic impurities, which are desorbed from the chemical filter itself, into the air passing through. Many of these chemical filters have been originally developed to remove noxious gases or offensive odor in the residential environment, and are diverted into the chemical filter for the clean room. Consequently, its performance and specification are designed to meet the standard of the field of residential environment, and it is inadequate from the first place to apply it to the air filter use for removing very small quantity of gaseous inorganic impurities, which causes the contamination of the substrate surface during the production of the semiconductor device (LSI) or LCD.
A chemical filter using chemical-impregnated activated charcoal is disclosed, for instance, in the Japanese laid-open patent No. Sho 61-103518. This filter is made by immersing the base material made of urethane foam into aqueous solution containing powdered activated charcoal, emulsion type adhesive and solid acid, and being dried. However, in this chemical filter, the desorption of gaseous organic substances occurs from synthetic rubber latex type and other water-dispersion type organic adhesive as an emulsion type adhesive, and from the base material of the urethane foam itself.
On the other hand, the so-called chemical filter utilizing the ion-exchange fiber has a following disadvantage. Gaseous organic impurities, generated from the binder and adhesive used for processing ion-exchange fiber based non-woven fabric, paper, felt into filter material having excellent permeability, and generated from the sealant used for fixing the filter material to the surrounding frame, are mixed with the air once cleaned by passing through the chemical filter and would give a negative influence to the manufacture of semiconductor device. Still other disadvantage is the generation of dust from the filter material.
In Japanese Patent Laid-open No. Hei 6-63333 and in Japanese Patent Laid-open No. Hei 6-142439, a so-called chemical filter using an ion-exchange fiber is disclosed.
In the former so-called chemical filter used for an air cleaning system utilizing ion-exchange fiber, the material of the chemical filter is structured by mixing cation-exchange fiber which is polyacrylonitrile series fiber having carboxylic group or anion-exchange. fiber which is vinylon series fiber having quaternary ammonium group, with glass fiber and heat adhesive fibrous binder. Accordingly, the desorption of gaseous organic substances occurs from various additives contained in the constituent polymer fiber, or a part of the ion-exchange group may desorb as carboxylic acid or ammonia.
Furthermore, in the latter so-called chemical filter used for cleaning up a very small quantity of contaminated air in a clean room, utilizing ion-exchange fiber, the ion-exchange fiber made by introducing sulfonic group, carboxylic group or phosphoric group as a cation-exchange group into non-woven fabric of polyethylene or polypropylene, or made by introducing strong basic quaternary ammonium group or weak basic group containing lower amine is utilized. Accordingly, the desorption of gaseous organic substances occurs from various additives contained in the constituent polymer non-woven fabric, or a part of the ion-exchange group may desorb as sulfonic acid, carboxylic acid, phophoric acid, ammonia, or amine.
Furthermore, when the air, for instance, in the photolithography process, which contains basic impurities such as ammonia, and gaseous organic impurities such as HMDS or trimethyl silanole at the same time, is treated, there exists complexity in such that the adsorptive material for removing basic impurities and the adsorptive material for removing organic impurities are separately prepared to make one adsorptive layer by mixing these two kinds of the adsorptive materials for the use, or the adsorptive layer for removing basic impurities and the adsorptive layer for removing organic impurities are made separately from two kinds of the adsorptive materials and are overlapped for the use.
If atmosphere in a clean room where four processes in Table 1 are performed, is mixed without being separated, as in the case when a plurality of processes are carried out in a big clean room, there arises a following disadvantage. For instance, in a process sensitive to inorganic impurities, if the gaseous impurities are removed by using the conventional chemical filter, the quality of the substrate is improved. This is because, in this process, the gaseous organic impurities generated from the chemical filter itself do not give negative influence to the quality of the substrate. However, in another process where the gaseous organic impurities are sensitive to organic impurities existing in the same room, surface contamination of the substrate is brought about, thereby causing the deterioration of the quality.
Therefore, in a big clean room where a plurality of processes are carried out, the chemical filter for the purpose of removing gaseous inorganic impurities is not only required to have superb property of removing these gaseous inorganic impurities, but also required not to generate gaseous organic impurities from the chemical filter itself. The development of a chemical filter which can remove at the same time both gaseous inorganic impurities and gaseous organic impurities, which cause the surface contamination of the substrate in the clean room atmosphere, has long been waited for.
In a case, as the photolithography process, where air containing both basic inorganic impurities and organic impurities is treated, hitherto, a chemical filter using activated charcoal for removing organic impurities was required in addition to a chemical filter for removing basic inorganic impurities. Accordingly, an improvement upon this point is also longed for.
Taking the above consideration into account, when a chemical filter is applied to a high efficiency air cleaning apparatus such as a clean room and a clean bench which can protect from acid contamination, basic contamination and organic substance contamination, the important issues are as follows.
The chemical filter itself should not be a contamination source of gaseous impurities and particulate impurities. (that is, not to be the secondary source of contamination)
The pressure loss of the chemical filter should be low.
The capability to remove gaseous impurities is high, and long-lived.
The conventional filter can not be said to satisfy these demands.
Accordingly, an object of the present invention is to provide an air filter which is safe and reliable from the standpoint of disaster prevention, does not desorb gaseous organic impurities and gaseous inorganic impurities from the filter itself, is able to remove not only gaseous basic impurities which cause problem particularly in the photolithography process but also gaseous organic impurities at the same time, and is able to prevent the substrate from surface contamination by gaseous basic impurities and gaseous organic impurities. Another object of the invention is to provide a method for manufacturing the filter as mentioned above. Still another object of the invention is to provide a high efficiency air cleaning apparatus equipped with the filter as mentioned above.
A filter to clean the air according to the present invention has an inorganic material layer, in which an adsorbent described below is fixed on the surface of a supporter using inorganic powder as a binder. The adsorbent is prepared by impregnating salt of an inorganic acid to powder of an inorganic substance that contains at least one of the following substances: diatom earth, silica, alumina, a mixture of silica and alumina, aluminum silicate, activated alumina, porous glass, activated clay, activated bentonite, or synthetic zeolite.
The air filter of the present invention has an inorganic material layer in which the adsorbent is fixed on the surface of the supporter using powder of the following inorganic substance xcex1 as a binder or a filler. The inorganic substance xcex1 consists of at least one of talc, kaolin mineral, bentonite, diatom earth, silica, alumina, a mixture of silica and alumina, aluminum silicate, activated alumina, porous glass, hydrated magnesium silicate clay mineral having a crystal structure of the ribbon type, activated clay, activated bentonite or synthetic zeolite.
xe2x80x9cThe fillerxe2x80x9d in this invention is an adsorbent and a component which constitutes an inorganic material layer. The filler fills up between particles of the adsorbent, but is arranged to keep air-permeable gaps or pores between the adsorbent. The inorganic substance xcex1 acts to fix the adsorbent to the supporter or fix among the particles of the adsorbent. However, the inorganic substance xcex1 does not have a capability to assure permanent adherence. Accordingly it becomes necessary to use an inorganic adhesion assisting agent such as silica sol or alumina sol in order to secure the adherence. Though the words xe2x80x9cbinderxe2x80x9d and xe2x80x9cfillerxe2x80x9d are used differently according to the degree of adherence in the adsorbent, the material itself is defined in the aforementioned list of the inorganic substance xcex1.
In the air filter according to the present invention, a first inorganic material layer and a second inorganic material layer consisting of the following substance are stacked on the surface of a supporter in a manner that either one of the first inorganic material layer or the second inorganic material layer is directly fixed to the surface of the supporter. The first inorganic material layer is a layer formed of the adsorbent fixed by inorganic substances as a binder. The second inorganic material layer is a layer consisting of at least one of diatom earth, silica, alumina, a mixture of silica and alumina, aluminum silicate, activated alumina, porous glass, hydrated magnesium silicate clay mineral having a crystal structure of the ribbon type, activated clay, activated bentonite or synthetic zeolite.
In the air filter of the present invention, pellets of the adsorbent are fixed on the surface of the supporter, using powder of the inorganic substance xcex1 as a binder or a filler.
The air filter comprises the supporter, a first pellet and a second pellet, wherein the second pellet is formed around the following first pellet by coating with powder of the following inorganic substance xcex2, and the second pellet is fixed on the surface of the supporter. The first pellet is prepared by pelletizing an adsorbent by using powder of an inorganic substance as a binder. The inorganic substance xcex2 is composed of at least one of diatom earth, silica, alumina, a mixture of silica and alumina, aluminum silicate, activated alumina, porous glass, hydrated magnesium silicate clay mineral having a crystal structure of the ribbon type, activated clay, activated bentonite or synthetic zeolite.
In the air filter according to the present invention, either a pellet which is formed by pelletizing the adsorbent using powder of the inorganic substance xcex1 as a binder or a filler, or a second pellet formed by coating with powder of the inorganic substance xcex2 around a first pellet which is formed by pelletizing the adsorbent using powder of an inorganic substance as a binder, is filled up in the casing.
In the air filter according to the present invention, the average pore diameter of the inorganic substance powder impregnated with salt of inorganic acid is 100 angstroms or more. In addition, sulfate is preferable as the salt of the inorganic acid. The supporter which has a honeycomb structure is preferred. The honeycomb structure is preferably made of inorganic fiber as an essential component.
A method for manufacturing the air filter has the following steps: immersing a supporter in a suspension in which the following adsorbent and powder of the following inorganic substance xcex1 are dispersed; and then drying the supporter, thereby forming the inorganic material layer fixed to the surface of the supporter. The adsorbent is prepared by impregnating salt of an inorganic acid to powder of an inorganic substance consisting of at least one of diatom earth, silica, alumina, a mixture of silica and alumina, aluminum silicate, activated alumina, porous glass, activated clay, activated bentonite, or synthetic zeolite; and the inorganic substance xcex1 is at least one of talc, kaolin mineral, bentonite, diatom earth, silica, alumina, a mixture of silica and alumina, aluminum silicate, activated alumina, porous glass, hydrated magnesium silicate clay mineral having a crystal structure of the ribbon type, activated clay, activated bentonite or synthetic zeolite.
A method for manufacturing the air filter according to the present invention comprises the steps of: immersing a supporter in a suspension in which the adsorbent and powder of an inorganic substance for the binder are dispersed; then drying the supporter, thereby forming a first inorganic material layer on the surface of the supporter; immersing the supporter on which the first inorganic material layer is formed in a suspension in which powder of the following inorganic substance xcex2 is dispersed; and then drying the supporter, thereby forming a second inorganic material layer on the surface of the first inorganic material layer, or forming the second its inorganic material layer on the surface of the supporter and then forming the first inorganic material layer on the second inorganic material layer. The inorganic substance xcex2 is at least one of diatom earth, silica, alumina, a mixture of silica and alumina, aluminum silicate, activated alumina, porous glass, hydrated magnesium silicate clay mineral having a crystal structure of the ribbon type, activated clay, activated bentonite or synthetic zeolite.
To the above method for manufacturing the air filter, a step of dissolving the salt of the inorganic acid in advance into the suspension separately before immersing the supporter, may be added.
A method for manufacturing the air filter according to the present invention comprises the steps of: preparing an adsorbent by impregnating salt of inorganic acid in advance to powder of the following inorganic substance xcex3; immersing a supporter in a suspension in which powder of the adsorbent and powder of the inorganic substance xcex1 as a binder are mixed together; and pulling up the supporter from the suspension for drying after immersing. The inorganic substance xcex3 is at least one of diatom earth, silica, alumina, a mixture of silica and alumina, aluminum silicate, activated alumina, porous glass, activated clay, activated bentonite or synthetic zeolite. In this case, a step of dissolving the salt of the inorganic acid separately into the suspension may be added. This method is effective when salt of inorganic acid having a poor solubility in the suspension, that is, not easy to dissolve into the suspension, is used.
The method for manufacturing the air filter comprises the steps of immersing the supporter in a suspension in which powder of the inorganic substance xcex3 and the inorganic substance xcex1 as a binder, are mixed together; pulling up the supporter from the suspension for drying after immersing in the suspension; immersing the dried supporter in a salt solution of inorganic acid; and pulling up the supporter from the solution for drying after immersing in the solution. This method is effective when salt of inorganic acid having a good solubility in the suspension, that is, easy to dissolve into the suspension, is used. In addition, since powder of the inorganic substance, that is, the inorganic adsorbent, is dried after fixing, the adsorber is in a good adsorptive state, so that the adsorbent with a large quantity of impregnated salt of the inorganic acid can be obtained. In the present invention, a holder of chemicals to which the salt of inorganic acid is impregnated is referred to as an adsorber, and an adsorber holding salt of inorganic acid is referred to as an adsorbent.
The method for manufacturing the air filter comprises the steps of: immersing the supporter in a suspension in which powder of the inorganic substance xcex3 and the inorganic substance xcex1 as a binder are mixed in a salt solution of inorganic acid; and pulling up the supporter from the suspension for drying after immersing in the suspension. In this case, since the adsorber and the salt of the inorganic acid are not separately impregnated, the number of the steps in the process can be reduced.
In the method for manufacturing each air filter according to the present invention, sulfate may be used as salt of inorganic acid.
A high efficiency air cleaning apparatus according to the present invention comprises: an air circulating path which circulates air in a space where clean atmosphere is required; an air filter disposed in the air circulating path; and a filter for removal of particulate impurities, arranged on the upstream side of the space in the air circulating path and on the downstream side of the air filter, and the air filter includes an inorganic material layer in which the following adsorbent is fixed on the surface of the supporter using powder of inorganic substance as a binder. The adsorbent is prepared by impregnating salt of an inorganic acid to powder of an inorganic substance consisting of at least one of diatom earth, silica, alumina, a mixture of silica and alumina, aluminum silicate, activated alumina, porous glass, activated clay, activated bentonite, or synthetic zeolite. The air filter used in the high efficiency air cleaning apparatus according to the present invention may include an inorganic material layer in which the adsorbent is fixed on the surface of the supporter using powder of the following inorganic substance xcex1 as a binder or a filler. The inorganic substance xcex1 is at least one of talc, kaolin mineral, bentonite, diatom earth, silica, alumina, a mixture of silica and alumina, aluminum silicate, activated alumina, porous glass, hydrated magnesium silicate clay mineral having a crystal structure of the ribbon type, activated clay, activated bentonite or synthetic zeolite. The first inorganic material layer is a layer formed of the adsorbent using powder of an inorganic substance as a binder. The second inorganic substance layer is a layer consisting of at least one of diatom earth, silica, alumina, a mixture of silica and alumina, aluminum silicate, activated alumina, porous glass, hydrated magnesium silicate clay mineral having a crystal structure of the ribbon type, activated clay, activated bentonite or synthetic zeolite.
The air filter, used in the high efficiency air cleaning apparatus of the invention, may use a layer consisting of a first inorganic material layer and a second inorganic material layer stacked on the surface of the supporter in a manner that either one of the first inorganic material layer or the second inorganic material layer is directly fixed to the surface of the supporter and the remaining inorganic material. layer is additionally fixed to the surface of the first fixed inorganic material layer so that the first inorganic material layer and the second inorganic layer are stacked on the surface of the supporter. The first inorganic material layer is a layer formed of the adsorbent fixed by powder of an inorganic substance as a binder. The second inorganic material layer is a layer consisting of at least one of diatom earth, silica, alumina, a mixture of silica and alumina, aluminum silicate, activated alumina, porous glass, hydrated magnesium silicate clay mineral having a crystal structure of the ribbon type, activated clay, activated bentonite or synthetic zeolite.
The air filter used in the high efficiency air cleaning apparatus according to the present invention may comprise a supporter and a layer formed of a plurality of pellets fixed to the surface of the supporter, wherein the pellet is formed by pelletizing the adsorbent, using powder of the inorganic substancea as a binder or a filler. The air filter used in the high efficiency air cleaning apparatus according to the present invention, may comprise a supporter, a first pellet, and a second pellet, wherein the second pellet is formed around the following first pellet by coating with powder of the following inorganic substance xcex2 and the second pellet is fixed on the surface of the supporter. The first pellet is made by pelletizing the adsorbent using powder of an inorganic substance as a binder. The inorganic substance xcex2 consists of at least one of diatom earth, silica, alumina, a mixture of silica and alumina, aluminum silicate, activated alumina, porous glass, hydrated magnesium silicate clay mineral having a crystal structure of the ribbon type, activated clay, activated bentonite or synthetic zeolite.
The air filter used in the high efficiency air cleaning apparatus may be formed by filling in a case either one of the following two types of pellets: the first pellets are formed by pelletizing the adsorbent using powder of the inorganic substance xcex1 as a binder or a filler, and the second pellets formed by coating with powder of the inorganic substance xcex2 around the first pellets made by pelletizing the adsorbent using powder of the inorganic substance as a binder.
In each of the above described high efficiency air cleaning apparatus, the air filter and a filter for removal of particulate impurities may be installed onto the ceiling portion of said space or room.
In the filter according to the present invention, at a portion in the inorganic material layer, where powder of the adsorbent and particulates of powder of the inorganic binder are adjacent to each other, gaps which serves as air permeable pores are formed. The object gas comes inside of the inorganic material layer from the surface of the inorganic material layer, or comes out from the inside of the inorganic material layer to the outside, both passing through the permeable pores. Thus, when the object gas passes through the inside of the inorganic material layer, the basic impurities in the object gas are adsorbed and removed by the adsorbent.
The air filter according to the present invention, in which a salt of an inorganic acid is impregnated to an adsorber, is composed solely of inorganic substances, and does not contain any organic substance which may decompose when placed in a high temperature atmosphere or placed with acid chemicals, not to mention flammable material such as activated charcoal. Accordingly, the filter according to the present invention is superior in a disaster prevention standpoint of view. Moreover, baking at a high temperature after impregnating with acid chemicals such as salt of inorganic acid, becomes possible. Accordingly, impurity gas which causes a problem when applied to a clean space, can be desorbed in advance. As a result, when air is passed through the filter at a speed of 1 m/s, the amount of the outgas from the air filter can be reduced to less than 1 ppb.
Life of the conventional adsorbent made of activated charcoal and the inorganic adsorbent used in the air filter of the present invention is considered. First, the measurement result of each impregnation amount of the salt of inorganic acid to activated charcoal and the inorganic substances will be shown in Table 2. The impregnation amount with various kinds of the salts of the inorganic acids per unit weight of the activated charcoal and the inorganic substance are shown in percentage by weight. The weight of the charcoal and the inorganic substance are weighed in a dry state without containing moisture. The charcoal and the inorganic substance are immersed for 30 minutes in an aqueous solution of various kinds of the salts of the inorganic acids having the concentration from 24% to 28%, and after pulling them up, they are dried in the air for two hours at 120xc2x0 C. Then, the impregnation amounts of the various salts of the inorganic acids are measured. xe2x80x9cActivated charcoal particulate Shirasagi GX for gas phase adsorptionxe2x80x9d (Trade name, manufactured by TAKEDA YAKUHIN Co., Ltd.) is used. The specific surface area of the activated charcoal is 1200 m2/g, and the pore volume is 0.86 cc/g. The specific surface area and the pore volume of silica, alumina series adsorber, and synthetic zeolite are shown in Table 5 which will be described later.
Note), The weight of activated charcoal or inorganic substance is the weight in a dry state without containing moisture.
As shown in Table 2, the inorganic substance such as silica, alumina series adsorber, synthetic zeolite can be much more increased in the impregnation amount than charcoal. Accordingly, series of the inorganic adsorbent such as the air filter of the present invention has an adsorptive life for gaseous basic impurities longer than the activated charcoal adsorbent.
Next, the consideration on the point of outgas will be described. In the chemical filter formed in a felt shape by combining the fibrous activated charcoal impregnated with chemical such as salt of inorganic acid etc., with organic binder such as polyester having a low melting point or polyester non-woven fabric, or in the chemical filter formed in a block shape and a sheet shape by firmly fixing the particulate charcoal impregnated with chemicals to a urethane foam or non-woven fabric using an adhesive, the outgas of organic impurity gas is generated from organic constituent itself in the organic binder, the urethane foam, the non-woven fabric, and the adhesive. The outgas of organic impurity gas is generated also from the reaction between the organic constituent and the impregnated acid of the chemicals.
For the reduction of the outgas, the so-called baking in which the whole filter is heated at a high temperature is effective. However, these organic constituents themselves start dissolving by heating only at 100xc2x0 C. to 150xc2x0 C., before 390xc2x0 C. which is the ignition point of activated charcoal, which makes it impossible to use for a filter. Additionally, the filter having activated charcoal ignites nearly at 380xc2x0 C. which is the decomposition temperature of zirconium sulfate when zirconium sulfate is used as a salt of inorganic acid, which makes it impossible to raise the temperature as high as 350xc2x0 C.
On the other hand, in the air filter of the present invention which consists solely of inorganic materials, the whole supporter formed of the inorganic material layer before impregnation with salt of inorganic acid can be baked at a high temperature above 350xc2x0 C., so that the outgas generated from the impurities contained in the inorganic material layer can be completely removed by this baking treatment. Each powder usable for the present invention, shown in Table 5 which will be described later, does not change in pore property so much when it is heated to 350xc2x0 C., thus, the heating to 350xc2x0 C. does not give negative influence to the performance and the specification as a filter.
A conventional chemical filter formed in a felt shape by combining fibrous activated charcoal impregnated with phosphoric acid, and organic binder such as low melting point polyester and polyester non-woven fabric, and an air filter produced in such that the whole honeycomb supporter formed out of an inorganic material layer consisting of, for instance, silica gel as the supporter of the present invention, is heated to 350xc2x0 C. for one hour, then immersed in an aqueous 27 wt % solution of aluminum sulfate 14 to 18 water (Trade name xe2x80x9cShika Tokkyuxe2x80x9d, manufactured by KANTOH KAGAKU Co., Ltd.) for 30 minutes. After pulling it up, it is dried for 2 hours under a current of air at 120xc2x0 C. Table 3 shows soluble outgas concentration detected with an ion-chromatography when purified air in a rated air quantity is allowed to pass through these air filters. The measurement result of the concentration of impurity gas contained in the purified air is shown as a background value.
According to the result, by using the air filter relating to the present invention, generation of the outgas from the filter itself can be confirmed to be far less than that by the conventional activated charcoal filter. Acetic acid and formic acid in the table are considered to be organic acid formed by the reaction between the organic constituents contained in the filter and the impregnated acid of the chemicals. As for NH3, chlorine, and NO2, they are considered to be derived from the impurities originally contained.
Next, the method for manufacturing will be discussed. The filter uses an inorganic adsorber according to the present invention, and when the filter kept dry after dehydrated is immersed in the chemical solution (solution of salt of inorganic acid) under atmospheric pressure circumstances, the chemical solution easily penetrates into the inside of the pore due to its hygroscopic property. On the other hand, the activated charcoal is hydrophobic in nature, and the chemical solution is hard to penetrate into the pore. Accordingly, in the previously known art, the pressure reduction/pressure application process was necessary in order to carry out the penetration. Therefore, the impregnated amount of chemicals by immersion under atmospheric pressure, is far more with the inorganic adsorber than with activated charcoal adsorber as shown in Table 2.
Table 4 shows the impregnation amount of various kinds of salts of inorganic acids for the following each case.
First case is that powder of silica gel, that is an inorganic substance, is dried in the air at 120xc2x0 C. for one hour to reduce the water content less than 1% of the total weight, and is kept in a dry box.
The second case is that the silica gel powder having water content about 30% of the total weight is kept long in a normal room atmosphere (about 20xc2x0 C., relative humidity 50%).
Each of the above silica gel is immersed in aqueous solution of various kinds of salts of inorganic acids having concentration from 24 wt % to 28 wt % for 30 minutes and is pulled up. Then it is dried in the air at 120xc2x0 C. for 2 hours.
As apparent from Table 4, the impregnation amount of various salt of inorganic acid can be increased by 25% to 33% when the silica gel is dehydration-treated compared with the case without dehydration. This is because water in the pore of the silica gel is driven out by the dehydration treatment, so that the solution can be easily penetrated into the pore on immersing in the solution of the chemicals.
The reason why the salt of the inorganic acid is used in place of the organic acid as the impregnation chemicals to the adsorber is as follows. The organic acid is called carboxylic acid, and the number of compounds having carboxylic group (xe2x80x94COOH) is said to be from a few thousand kinds to tens of thousands kinds. If carboxylic acid desorbs as a gaseous material by any chance, it becomes an organic contamination material. In addition, when an air filter containing the organic acid is used for a long time in a clean room, even if there is no gasification of the organic acid itself, there arises a fear that the organic acid is gradually decomposed into a substance like acetic acid which is easy to gasify, by breeding of a microorganism which takes the organic acid as a nutritive source, thereby causing new contamination by the gaseous decomposition material. Furthermore, once the organic acid decomposes, its initial function as the organic impregnation chemicals to neutralize and chemically adsorb the gaseous basic impurities is lost. On the other hand, salt of inorganic acid can not be a nutritive source for a microorganism, and, salt of inorganic acid is known to have a sterilizing function against the microorganism. Accordingly, for the impregnation material to the adsorber used in the clean room, a salt of an inorganic acid is more preferable than an organic acid.
On the other hand, there are many inorganic acids which are hazardous materials to human body if they have high acidity (for instance, hydrochloric acid, sulfuric acid, nitric acid, etc.). In addition, when an air filter containing strongly acidic inorganic acid is left in an environment as a waste, the inorganic acid dissolved in a rain-water causes water pollution. Consequently, a salt of an inorganic acid which is small in acidity and less hazardous to the human body, is suitable as the acid substance to impregnate to the adsorber.
There are two kinds of zeolite, natural zeolite and synthetic zeolite, and industrially synthetic zeolite is more widely used than natural zeolite which is poor in purity. Natural zeolite has pores in various sizes from 3 to 10 angstroms, but it is possible for the synthetic zeolite to produce pores uniform in size.
In the present invention, when synthetic zeolite is used as the material to be filled between individual particles of the adsorbent, the individual zeolite particles are filled between individual particles of the adsorbent in which a salt of an inorganic acid is impregnated to powder of the inorganic substance, and gaps to be air-permeable pores are formed in the adjacent portions between the adsorbent particles and zeolite particles. The air-permeable pore has a function in a manner that the object air to be cleaned can reach the salt of the inorganic acid impregnated to the adsorbent particle.
The pores which the synthetic zeolite itself possesses, adsorb and remove gaseous organic impurities with particle diameter smaller than the size of the pores, when the object air to be cleaned passes through the. air-permeable pores. Accordingly, if any synthetic zeolite is selected, which has a suitable pore size according to the kind of the gaseous organic impurities contained in the object air to be cleaned, it is possible to design an air filter having excellent performance to adsorb gaseous organic impurities.
As the silica of the present invention silica gel may be used, while alumina gel may be used as alumina. Similarly, the mixture of silica gel and alumina gel may be used as the mixture of silica and alumina.
In the present invention, as for the constituent having a pellet, gaps to be air-permeable pores, are formed in the portions where particulates of powder of the adsorbent or particulates of powder of the inorganic binder are adjacent to each other, and the object gas to be cleaned passes through the air-permeable pores from the surface of the pellet to come into the inside of the pellet or to come out from the inside of the pellet to the outside. Then, the basic gaseous impurities in the air are adsorbed and removed by the adsorbent.
In the present invention, gaseous basic impurities in the clean room air can be adsorbed and removed by means of the adsorbent. And gaseous organic impurities such as DOP (di-octyl phthalate), DBP (di-butyl phthalate), BHT (butyl hydroxytoluene) siloxane and so forth, can be adsorbed and removed by properly selecting powder of an inorganic substance as a binder, which includes micropores and/or mesopores. Accordingly, the filter according to the invention is applicable to even the atmosphere including basic gaseous impurities and gaseous organic substances, which cause the surface contamination of the substrate, and may capture most of these chemical contaminating substances.
In the present invention, as for a filter having a first inorganic material layer and a second inorganic material layer, the exfoliation from the salt of the inorganic acid contained in the first inorganic material layer in the inside of the second inorganic material layer or contained in the first pellet can be prevented by the outer second inorganic layer or the coating layer outside the second pellet, in the case where the first inorganic material layer is placed in the inside, or in the case when the filter is provided with the second pellet. In this case, if powder of inorganic substance forming the second inorganic material layer or the outermost coating layer has pore suitable for physical adsorption of gaseous organic substances such as DOP, DBP, BHT, siloxane, and so forth which cause the surface contamination of the substrate, there exists another advantage in such that these gaseous organic substances can be adsorbed and removed by the pores.
When the first inorganic material layer is placed on the outer side, though the aforementioned dust protection effect vanishes, the adsorption effect for the gaseous impurities remain unchanged. The adsorption and removal of the basic gaseous impurities in the air is performed in the outermost first inorganic material layer which are in direct contact with the air in the clean room, and the adsorption and removal of the gaseous organic impurities in the air is performed in the inner second inorganic material layer.
Here, the aforementioned adsorbent will be briefly explained. The adsorbent is prepared by impregnating a salt of an inorganic acid to powder of an inorganic substance consisting of at least one of diatom earth, silica, alumina, a mixture of silica and alumina, aluminum silicate, activated alumina, porous glass, activated clay, activated bentonite, or synthetic zeolite. For powder of these inorganic substance, Table 5 (I) shows the measurement result by means of the gas adsorption method for the total volume of the pores distributing in the range from 5 to 300 angstroms and the specific surface area of the pores of the inorganic substance.
Silica gel is used as silica for powder sample. Alumina gel is used as alumina for powder sample. The reason to pay attention to pores distributing in the range from 5 angstroms to 300 angstroms is that the pore having size of this range is most suitable for the physical adsorption of the gaseous impurities, and the gaseous basic impurities contained in the object air are apt to be adsorbed in these pores. Since the salt of the inorganic acid is impregnated on the surface of the pore, the gaseous basic impurities neutralize with the salt of the inorganic acid at once to be chemically adsorbed, so that they never desorb again. The larger the specific surface area of powder of the inorganic substance is, the larger the surface to be impregnated with the salt of the inorganic acid and the volume to absorb the gaseous impurities becomes.
However, when the inorganic substance in Table 5 (I) is impregnated with salt of inorganic acid, the salt of the inorganic acid comes into the gap portion of the pore. Accordingly, the specific surface area and the pore volume are considerably decreased when compared with the pure inorganic substance before the impregnation. No inorganic substance in Table 5 (I) initiates a chemical reaction with the impregnated salt of the inorganic acid, so that the capability of chemical adsorption to the gaseous basic impurities in the salt of the inorganic acid is not negatively effected. Diatom earth is hydrous colloidal silicic acid and hard to react with acid. Silica, alumina, a mixture of silica and alumina, aluminum silicate, and porous glass are also hard to react with acid. Activated alumina is obtained by heat dehydration (450xc2x0 C.) of aluminum hydroxide and is hard to dissolve in acid. Activated clay can be obtained by sulfuric acid treatment of acid clay and is hard to dissolve in acid. Activated bentonite is obtained when calcium type bentonite or acid clay is treated by heated sulfuric acid to eluate aluminum montmorillonite or magnesium montmorillonite, so that excess silicic acid having many pores on the surface thereof is produced. Accordingly, its adsorptive power and the catalytic power are increased and it is hard to be affected by acid. Synthetic zeolite also has a nature not to be easily affected by acid.
On one hand, in all inorganic substances such as talc, kaolin mineral, and bentonite, which are generally used as a binder, as shown in Table 5 (II), the pore volume and the specific surface area of powder distributing in the range from 5 to 300 angstroms are extremely small compared with the inorganic substances in Table 5 (I). The pore volume and specific surface area of powder of sepiolite which is a kind of hydrated magnesium silicate clay minerals having a crystal structure of the ribbon type in Table 5 (III), ranging from 5 to 300 angstroms are large, similar to the inorganic substance in Table 5 (I). However, if the inorganic substance in Table 5 (II) and (III) is impregnated with a salt of an inorganic acid, the hydroxyl group and salt of inorganic acid neutralize with each other like talc (Mg3[(OH)2Si4O10], kaolin mineral (Al4[(OH)8Si4O10]) or sepiolite (Mg8Si12O30(OH2)4 6 to 8H2O) or ion-exchange with each other, or chemically react with salt of inorganic acid by means of the cation exchange function like bentonite. Accordingly, the chemical adsorption function of the salt of the inorganic acid to the gaseous basic impurities is lost.
Since powder of the aforementioned adsorbent (the adsorbent prepared by impregnating the salt of the inorganic acid to powder of the inorganic substance consisting of at least one of diatom earth, silica, alumina, a mixture of silica and alumina, aluminum silicate, activated alumina, porous glass, activated clay, activated bentonite, or synthetic zeolite) has no self-bonding power, in order to pelletize powder of the adsorbent or to fix it on the surface of the supporter in a layer, a binder must be added.
In the present invention, an inorganic material layer is formed by fixing powder of the adsorbent on the surface of the supporter, or a pellet is formed by pelletizing powder of the adsorbent, using powder of an inorganic substance consisting of at least one of talc, kaolin mineral, bentonite, diatom earth, silica, alumina, a mixture of silica and alumina, aluminum silicate, activated alumina, porous glass, hydrated magnesium silicate clay mineral having a crystal structure of the ribbon type, activated clay, activated bentonite or synthetic zeolite as a binder or a filler. Silica may be replaced by, for instance, silica gel, while alumina may be replaced by, for instance, alumina gel. Similarly, the mixture of silica and alumina may be replaced by, for instance, the mixed gel consisting of silica gel and alumina gel.
It is preferable that the supporter for use in the filter according to the invention is made in a form of a honeycomb structure. It is also preferable that the honeycomb structure is formed of inorganic fiber as the indispensable requirement therefore. The adsorptive layer of the honeycomb structure has a ceramic-like solid surface and the amount of exfoliation is extremely small compared with the conventional chemical filter using charcoal impregnated with chemicals or ion exchange fiber. When powder of each inorganic substance is applied to powder of inorganic substance used for fixing the salt of the inorganic acid, or the binder used for fixing thereof on the surface of the supporter in the adsorbent, there is hardly any desorption of gaseous organic substance from the material constituting the air filter of the present invention.
The term xe2x80x9choneycomb structurexe2x80x9d as used in the invention stands for not only a structure with the shape of a honeycomb but also any structure including a plurality of cell as an element through which the air can pass, for instance, a structure having a grid shape in section, a structure formed of a plurality of corrugated sheets and so on. Furthermore, the term xe2x80x9csupporterxe2x80x9d is not limited to the honeycomb structure and may be any of a three dimensional mesh structure like rock wool. In this case, powder of the adsorbent of the present invention is preferably fixed not only on the surface of the mesh structure but also to the inside thereof.
As a method to fix the adsorbent to the surface of the supporter, there are a method to fix by a binder made of powder of an inorganic substance, and a method to pelletize powder of the adsorbent into pellets using powder of the inorganic substance as the binder and to adhere the pellet on the surface of the supporter with an inorganic adhesion assisting agent.
In the air filter according to the present invention, when either one of a pellet made by pelletizing the adsorbent using powder of the inorganic substance xcex1 as a binder or a filler, or a second pellet formed by coating with powder of the inorganic substance xcex2 around the first pellet made by pelletizing the adsorbent with powder of the inorganic substance, is filled in a casing, flexibility in design can be obtained so as to properly select the shape and size of the casing, and the amount of the pellet filled in the casing according to the shape and size of the flow path of the air, and the installation requirement of the filter.
According to the present invention, flexibility in design can be obtained so as to be able to properly select the shape and size of the casing, and the amount of the pellet filled in the casing according to the shape and size of the flow path of the air and the installation requirement of the filter.
In instances where both gaseous basic impurities and gaseous organic impurities are required to be completely removed, in the inorganic substance, the inorganic substance xcex1, and the inorganic substance xcex2 used as the binder of the adsorbent, the total pore volume of powder having the diameter in the range from 5 to 300 angstroms is preferably equal to or more than 0.2 cc/g, or the specific surface area of the pore is preferably equal to or more than 100 m2/g. Whatever the case may be, each powder of these inorganic substance, the inorganic substance xcex1, and the inorganic substance xcex2 contained in the inorganic material layer or the pellet used as a binder for powder of the adsorbent, has the capability to mechanically fix powder of the adsorbent on the surface. of the supporter, the capability to act as a binding agent for pelletizing powder of the adsorbent into a pellet, and also has a porous structure in such that the object gas to be cleaned can easily arrive at or return from the salt of the inorganic acid impregnated to the surface of the inorganic powder consisting the adsorbent. The object air to be cleaned can easily reach the salt of the inorganic acid impregnated to the surface of the adsorbent powder through the air-permeable pore or gap formed in the adjacent portion between each powder of the inorganic substance, the inorganic substance xcex1, and the inorganic substance xcex2 used as the adsorbent and the binder, so that gaseous basic impurities are removed. During this process, the gaseous organic impurities are physically adsorbed on the surface of the pores having a pore diameter in the above-mentioned range existing on the surface of the aforementioned inorganic powder used as a binder or a filler, and removed. Accordingly, besides the removal of gaseous basic impurities, which is one of the main purposes of the invention, the removal of gaseous organic impurities can be secondarily achieved by means of the pore of each inorganic powder described before which is used as a binder or a filler.
The specific surface area and the pore volume of the binder belonging to the groups (I) and (III) in Table 5 are considerably larger than those of the binder belonging to the group (II). Therefore, the binders (I) and (III) are superior to those of the group (II) in terms of the physical adsorption function against gaseous organic impurities. The inorganic binders of the group (II), for instance talc, kaolin mineral, and bentonite are selected and used primarily taking account of air permeability of the porous structure constructed therewith. Most of air permeable pores that are formed between adjacent binder particulates and also between the binder particulates and the adsorbent particulates fixed to the supporter, have a size of about 500 angstroms or more. In other words, these air permeable pores are excellent in air permeability, but are in the macropore range and less contribute to the physical adsorption. Furthermore, the particulate surface of the inorganic binder such as talc, kaolin mineral, and bentonite, includes few pores capable of performing the physical adsorption of gaseous organic impurities. By the way, the pore volumes of the pore size in the range from 150 to 150,000 angstroms measured by the liquid mercury press-in method for each powder of talc, kaolin mineral, and bentonite are 0.92 cc/g, 1.11 cc/g, and 0.49 cc/g, respectively. It is found that the group (II) have few micropores and mesopores relating to the physical adsorption, but mostly have macropores excellent in air permeability, when compared with Table 5.
Consequently, the inorganic binder of the group (II) in Table 5 is primarily used for the purpose of mechanically holding powder of the adsorbent on the surface of the supporter. The quantity of powder of the adsorbent to be held on the supporter surface is preferably increased as much as possible in order to sufficiently remove gaseous inorganic impurities. On the other hand, the quantity ratio of the inorganic binder to powder of the adsorbent is preferably as small as possible. However, if the quantity of the inorganic binder is reduced excessively, powder of the adsorbent is incompletely held on the supporter surface, which might cause powder of the adsorbent to peel off and make it act as a kind of a dust source. For instance, when forming the inorganic material layer by fixing powder of the adsorbent to the supporter surface, using bentonite as an inorganic binder along with silica sol as an inorganic adhesion assisting agent, if the weight ratio of powder of the adsorbent to the entire inorganic material layer on the supporter surface exceeds 75%, it is found that the mechanical strength for holding the inorganic material layer on the supporter surface is impracticably weakened. In short, the role of the inorganic binder of the group (II) is to firmly hold the inorganic material layer on the supporter surface and at the same time, to secure such excellent air permeability that the object gas can reach powder surface of the adsorbent with ease, but is not required to physically adsorb gaseous organic impurities.
On the other hand, in the case of the inorganic binder of the group (I) and (III) in Table 5, the air permeable pores that are formed between adjacent binder particulates and also between the binder particulate and the adsorbent particulate, have a size similar to that of the air permeable pore formed with the inorganic binder of the group (II). It should be noted that the pore volumes having the pore diameter in the range from 150 to 150,000 angstroms measured by the liquid mercury press-in method for powder in the group (II) are 0.8 cc/g, to 1.7 cc/g, respectively. The specific volume having the pore diameter in the range from 150 to 150,000 angstroms measured by the liquid mercury press-in method for sepiolite in the group (III) is 1.6 cc/g. Thus, the excellent air permeability is secured and the object gas can easily reach powder surface of the adsorbent, so that removal of gaseous basic impurities is carried out in the same manner as in the case of the binder belonging to the group (II).
The strength of the physical adsorption by the pore against gaseous molecules increases in the order of macropore, mesopore, and micropore. It is said in general that the macropore hardly contributes to the physical adsorption. The particulate of the inorganic binder of the group (I) (III) has on its surface the pore suitable for carrying out the physical adsorption against gaseous molecules, namely the micropore with the pore diameter of 20 angstroms or less, and the mesopore with the pore diameter in the range of 20 to 500 angstroms, so that gaseous organic impurities such as DOP, DBP, BHT, siloxsane which are hard to be removed by powder of the adsorbent and cause the surface contamination of the substrate, can be physically adsorbed and removed at powder surface of the binder particulates.
When the binder of the group (I) and (III) in table 5, the ratio (weight base) of the adsorbent powder to the inorganic material layer on the supporter surface has an upper limit value. For instance, when forming the inorganic material layer by fixing powder of the adsorbent to the supporter surface, using silica gel as an inorganic binder, if the weight ratio of powder of the adsorbent to the entire inorganic material layer exceeds 72%, it is found that the mechanical strength for holding the inorganic material layer on the supporter surface is impracticably weakened.
In the air filter of the invention, in which the adsorbent is fixed on the surface of the supporter using an inorganic powder of the groups (I) and (III) in Table 5 as a binder or a filler, and the air filter in which pellets made of the adsorbent is fixed on the surface of the supporter using the above-described powder as a binder, the supporter is coated with the inorganic material layer or the pellet layer to adsorb and remove gaseous organic impurities. Therefore, even if the supporter includes an organic material that desorbs gaseous organic impurities, the gaseous organic impurities generated from the supporter itself can be adsorbed and removed by the inorganic material layer or the pellet layer covering the supporter, so that the gaseous organic impurities are never mixed again with the once cleaned object air on the downstream side of the filter.
In the air filter according to the present invention, an average pore diameter of the adsorber made of the inorganic powder to which a salt of an inorganic acid is impregnated, is preferably 100 angstroms or more. The salt of the inorganic acids dissolve in the liquid according to the respective solubility, but they lose moisture content during drying process and remain in the adsorbent held by the supporter as the solid state salt of the inorganic acids with a very small moisture content. If the solid state salt of the inorganic acid blocks the pore opening of the adsorber, the gaseous basic impurities to be removed can not reach the gap in the pore. The salt of the inorganic acid fixed on the gap surface inside the pore blocking the opening thereof does not initiate neutralization reaction with the gaseous basic impurities and becomes an invalid component without any contribution to the adsorption. The inventor and others measure the protection efficiency of ammonia by the air filter in which various silica gel having different pore diameters, the pore being impregnated with a salt of an inorganic acid is held on the surface of the supporter, and obtain the result shown in Table 6.
As clear from the above result, if the average pore diameter of the adsorber is more than 100 angstroms, the opening of the pore is hard to be blocked by the salt of the inorganic salt, and the protection efficiency is nbt reduced.
In the filter of the present invention, either the inorganic material layer, the first inorganic material layer, the second inorganic material layer, the pellet, the first pellet, or the second pellet may include an inorganic adhesion assisting agent. The inorganic adhesion assisting agent is preferably the one selected from sodium silicate, silica, and alumina. In this case, silica sol may be used in place of silica and alumina sol may be used in place of alumina.
The salt of the inorganic acid used in the air filter of the present invention is preferably sulfate.
Main salts of inorganic acids when classified by the sort of the acid are carbonate, hydrochloride, sulfate, nitrate, and phosphate. Among them, carbonate is inferior in adsorption ability of ammonia due to too weak acidity. Hydrochloride might generate gases such as chlorine and hydrogen chloride which are corrosive to metal, thereby making it difficult to utilize. Nitrate produces ammonium nitrate by neutralizing with ammonia but there is danger to explode by heating in a closed state. Phosphate is often impregnated to a conventional filter using activated charcoal for removing gaseous basic impurities because it is weak in acidity and less harmful to the human body. However, when once phosphorous desorbs in a gaseous material, it becomes a contamination material to dopant in Table 1. Accordingly, phosphate is difficult to use in a high efficiency air cleaning apparatus. As above, sulfate is most preferable as the salt of the inorganic acid used in the present invention.
The boiling point of sulfate is preferably more than 120xc2x0 C. When an inorganic material layer is formed by fixing the adsorbent prepared by impregnating sulfate to powder of an inorganic substance on the surface of the supporter using powder of an inorganic substance as a binder, moisture in the inorganic material layer is evaporated and removed by heat-drying the supporter surface. In such a case, in order to promote water evaporation, the temperature is often set at 110xc2x0 C. to 120xc2x0 C. which is above 100xc2x0 C., the boiling point of water. From this point of view, the boiling point of the sulfate must be above 120xc2x0 C.
When an air filter having a long life is configured, it is preferable to use an adsorptive material having an ion exchange amount of more than 10 meq/g, because material having an ion exchange amount of less than 10 meq/g has a small adsorptive capacity for basic contamination as an adsorbent. Furthermore, since it is difficult for a substance which is liquid at an ordinary temperature to fix on an inorganic powder, it is preferable to use sulfate which is solid at an ordinary temperature. Among sulfates, substances which satisfy all these requirements are titanyl sulfate, aluminum sulfate, vanadium sulfate, zirconium sulfate, sodium hydrogen sulfate, and potassium hydrogen sulfate.
A method for manufacturing an air filter of the present invention, the salt of the inorganic acid may be dissolved separately in advance in a suspension (an adsorbent is contained as an essential component) to immerse the supporter therein. Then when the supporter is pulled up from the suspension, the salt of the inorganic acid dissolved in the suspension penetrates in the gap portion adjacent to the particulates of the adsorbent powder or powder of the inorganic binder. The supporter is dried thereafter, only moisture is evaporated from the salt of the inorganic acid immersed in the gap portion so that the gap portion becomes an air permeable pore in the inorganic material layer. In short, the solid matter of the salt of the inorganic acid is held on the wall surface inside the air permeable pore. As a result, it becomes possible to extend the life of the filter by increasing the amount of the salt of the inorganic acid; The salt of the inorganic acid dissolved in advance in the suspension has an effect to complement the salt of the inorganic acid lost by dissolving from powder of the adsorbent into the suspension.
According to a method for manufacturing the air filter of the present invention, the filter can be produced only from the materials not generating any gaseous organic impurities. In addition, the air filter can be practically composed only of materials having no flammable substance. Silica may be replaced by silica gel, while alumina may be replaced by alumina gel. Similarly, the mixture of silica and alumina may be replaced by the mixed gel consisting of silica gel and alumina gel. When an inorganic material layer is formed on the surface of the supporter, or when either one of the first inorganic material layer or the second inorganic layer is formed on the other, if a sol-state inorganic adhesion assisting agent is mixed in the suspension, the inorganic material layer, the first and second inorganic material layers will respectively contain the inorganic adhesion assisting agent.
As a salt of an inorganic acid used for the manufacturing of the air filter of the invention, sulfates are preferable as described before. Among them, titanyl sulfate, aluminum sulfate, vanadium sulfate, zirconium sulfate, sodium hydrogen sulfate, and potassium hydrogen sulfate are suitable.
According to the high efficiency air cleaning apparatus of the present invention, the surface contamination caused by the environment on the surface of the substrate to be handled in the space in the high efficiency air cleaning apparatus can be protected by removing the gaseous basic impurities contained in the circulating air in the high efficiency air cleaning apparatus and according to circumstances, by also removing gaseous organic impurities. The high efficiency air cleaning apparatus of the present invention uses neither flammable activated charcoal nor flammable ion exchange fiber, thereby making it superior in preventing fire. Accordingly, the air filter and the filter to remove particulate impurities can be placed on the ceiling portion of the space.